In balloon catheters of this type heating of heat medium, frequently a liquid which after insertion of the catheter balloon into the organ to be treated, is used to expand the balloon. The heating can take place by an element positioned in the balloon for the transfer of heat to the liquid and further on to the inner calls of the cavity, and in certain designs the liquid is brought to circulate in the expanded balloon by means of some form of pumping system. Alternatively, the heating medium can be externally heated and then introduced into the balloon. A number of different types of balloon catheters for heat treatment of body cavities are known, and in certain cases it has been suggested that the heated liquid is circulated within the balloon for reaching an even heat distribution in the expanded balloon and efficient heat transfer to surrounding tissues.
In U.S. Pat. No. 4,949,718 there is described a balloon catheter for the destruction of uterus mucosae by means of heating, the heating taking place through a liquid which is heated by means of an electric heating element comprising a spiral surrounded by a screen provided with holes to a temperature near or at the boiling point of the liquid. In this system a certain self-circulation will be obtained in view of the fact that the liquid is heated to a temperature near its boiling point. The disadvantage of this device is, however, that sudden formation of steam can increase the pressure in the cavity in an uncontrolled manner resulting in risk for rupture in the uterus muscle. Furthermore, no efficient heat transfer to the surrounding tissues will be obtained in view of lack of a forced circulation of the liquid.
In U.S. Pat. No. 4,160,455 there is described a device for the heat treatment of a body cavity where a heating medium in the form of a liquid is heated by electric resistance elements and is circulated within an enclosure for the distribution of heat and improvement of the heat transfer to the surrounding. The circulation is provided by means of a bellows system providing an oscillating movement, and by a system of back valves in a housing surrounding the heating element the oscillating movement of liquid is transferred to a circulating movement.
In PCT-application SE94/00208 there is described a balloon catheter for hyperthermal treatment of body cavities, for example uterus, where a balloon, after insertion into the cavity, is expanded by means of a liquid heating medium injected into the rear or proximal end of the catheter by means of for example a syringe. The heating device consists of an auto-regulating material having a so called Curie point and the energy supply is carried out in an electric way. This auto-controlling material may either be constituted by a ferromagnetic metal alloy which is wirelessly heated to the Curie point by a magnetic field affecting the material. The Curie point has been selected for providing the desired therapeutic effect. An alternative heating element according to this prior art is one wherein the auto-controlling material consists of a number of thin lamellae of so called PTC elements having a selected Curie point placed in the cylindric housing, heating taking place by means of an electric current under low voltage.
In the device according to said PCT-application the liquid heat medium is circulated in the balloon so that efficient heat transfer to surrounding tissue is provided, since otherwise the auto-controlling character of the material would result in shut down of the elements and thereby lack of release of sufficient power. Also in this case the circulation of the heat medium takes place by imparting to same a reciprocating movement which is then converted to circulation in the expanded balloon by the use of a system of counter-positioned back valves. Also in this case the circulation contributes to a more even heat transfer to the surrounding tissues, which is essential for providing the desired impact.
However, it has been found that the known devices are associated with serious disadvantages. Generally, these consist in the formation of air pockets at certain positions in the system when the device is filled with the liquid by which heating shall take place, and such air pockets when assembled in the balloon result in impaired heat transfer so that the desired effect will not be obtained. In such systems based on forced circulation, for example according to U.S. Pat. No. 4,160,455, air pockets result in substantially reduced pumping capacity in view of the compressibility of the air.